Hubbard Glacier, Alaska: 2002 closure and outburst of Russell Fjord and postflood conditions at Gilbert Point

Hubbard Glacier, the largest temperate tidewater glacier in the world, has been advancing since 1895 AD and has now twice dammed 60-km-long Russell Fjord, once in 1986 and more recently in 2002. This paper focuses on the 2002 event, when a strong spring advance pushed shallow submarine proglacial sediments against Gilbert Point, closing off Russell Fjord by late June. As a consequence, upstream ice flow decelerated from 5 m d(-1) to 1.5 m d(-1), with flow diverging to either side of Gilbert Point. Lake height reached 15 m asl before intense rains caused lake water to overtop the moraine dam on 14 August 2002. Three cubic kilometers of water were released within 30 hours, with peak discharge reaching 55,000 m(3) s(-1) 24 hours after the flood began. The discharge records for the 1986 and 2002 outbursts differ significantly and reflect differences in lake height (26 m versus 15 m) and dam types (ice versus moraine). The 2002 outburst proceeded in two stages: (1) relatively slow overtopping of the subaerial moraine with downward erosion rates of 1-2 m h(-1) with little lateral expansion, (2) followed by faster downward erosion of the submarine moraine (up to 7 m h(-1)) with rapid lateral expansion of the channel by ice calving (similar to 7 m h(-1)). The annual average terminus position at Gilbert Point has remained constant since 2002, although there are seasonal variations of 100-200 m. The deep channel, strong tidal currents, and seasonally warm ocean water appear to have prevented the advance of this segment of the terminus despite the glacier's continued advance elsewhere along its terminus. Sediments are slowly filling in the channel at a rate of about 4 m yr(-1), and their steady accumulation may eventually trigger the next closure.